1. Technical Field
This invention relates to refrigeration systems. Specifically this invention relates to systems for detecting an accumulation of frost on an evaporator and initiating a defrost cycle when the accumulation of frost is excessive.
2. Background Art
Conventional refrigeration systems achieve cooling by allowing a refrigerant such as ammonia or a fluorocarbon material to expand in the coils of an evaporator. As the refrigerant expands it absorbs heat from the surroundings. A fan or other air moving device is used to draw air through the evaporator so heat is more effectively removed from the air in the refrigerated space.
As the temperature in the evaporator is generally below the freezing point of water, water vapor in the air often condenses on the evaporator and solidifies as frost. The buildup of frost adversely effects the cooling efficiency of the evaporator due to two cooperating factors. First, frost is a thermal insulator. The thicker the frost layer on the evaporator coils, the less efficient the heat transfer between the air and the evaporator. In addition, the buildup of frost restricts the air flow through the evaporator coils. As a result, less air is cooled. Eventually, as frost builds up, the combined effects of reduced air flow and less heat transfer require that the evaporator be defrosted to restore cooling efficiency.
In prior systems evaporators have been defrosted automatically and periodically under timed control. The time between the defrost cycles is set by an operator based on their experience with the system. The problem with such timed systems is that the amount of water vapor in the air in the cool area varies depending on a number of factors. Some of these factors include the humidity in the environment surrounding the space being cooled, the number of times the access door to the cooled area is opened, and the duration of such openings. The temperature in the cooled area, the temperature of the evaporator, the velocity of the air passing through the evaporator and the evaporation of water from items stored in the cooled area also effect the rate of frost buildup. Usually timed defrost systems must be set for the most severe frost accumulation conditions. This leads to unnecessary defrost cycles which waste energy and cost money.
Several prior systems have been used for detecting the buildup of frost on the coils of evaporators so that defrost cycles are initiated only when the buildup is large enough to adversely impact the cooling efficiency of the refrigeration system. In U.S. Pat. No. 4,123,792 a system is described which measures the power consumed by an electric fan motor which draws air over the evaporator. The principle of operation of this system is that frost buildup on the evaporator impedes air flow. As frost builds the motor works harder to drive the fan and when a particular set point is reached a defrost cycle is initiated. The problem with such a system is that factors other than frost buildup also impact the power requirements for a fan motor. Such factors include the supply voltage, the temperature in the cooled space and the age of the motor. Other systems such as that shown in U.S. Pat. No. 4,400,949 have employed similar principles but have attempted to compensate for some of the factors which may give false frost buildup indications. The system disclosed in this patent monitors other parameters in an attempt to determine if factors other than frost buildup are causing increased power consumption. This system is complex and expensive to implement. It also has the disadvantage that the characteristics of refrigeration system components vary with age and loss of refrigerant. Such a system cannot compensate for these factors.
Other frost detection systems such as those shown in U.S. Pat. Nos. 4,045,971 and 4,232,528, employ photoelectric sensors to detect the level of frost buildup on an evaporator coil. The problem with photoelectric sensors is that they are only capable of sensing frost at a particular location on an evaporator. As frost buildup is not always regular or uniform, frost may build at locations away from the photo sensor and not be detected. This will cause the evaporator to operate inefficiently because defrosting is needed. In other situations frost may build up near the sensor to a greater extent than at other locations causing defrost to be initiated when it is not needed. Another deficiency of such systems is that they may not detect the buildup of transparent, clear ice.
Another prior art system manufactured by Levy Systems of Campbell, California, senses the differences in air temperature on each side of the evaporator in the refrigerated space as well as the temperature of the refrigerant leaving the evaporator. The data from the sensors is processed to determine if there is a frost buildup. The problem with the Levy system is that it is complex and that changes in temperature across the evaporator indicative of frost buildup may occur in other situations as well. In addition, this system cannot compensate for changes that occur with age or loss of refrigerant.
Thus, there exists a need for a frost detection system that is more accurate, reliable and less expensive to implement than existing systems and which is unaffected by changes in the system due to changes in system components.